This is a phase II, 12-week, randomized, double-blind, proof-of-concept study involving subjects with stable weight loss and no evidence of active opportunistic complications or malabsorption. Subjects will be randomized to receive either an oral nutritional supplement containing increased amounts of high quality protein (whey), which is rich in cyst(e)ine and glutamine, from a conventional source or from an isocaloric, identical tasting supplement without added whey protein. The supplements will be administered mid-morning and mid-afternoon between meals. The GCRC will be used for outpatient procedures: bioelectrical eimpedance analysis, diet history, anthropometry (standardized weight/height/body circumference measurements), blood draw, questionnaire.